The water and tea tree oil was a huge PITA for me. It always separated requiring shaking and it didn't spray nicely. I tried mixing it with some mild de-greasers and cleaners I used, thinking that if it was emulsified it might be a bit better. That reduced it's effectiveness as a cleaner, and it didn't really help the tea tree oil work any better. It seemed a bit silly to me spraying water everywhere when the whole point is to have a dry, clean smelling boat.
My current recipe for use in the bilges and lockers is really simple. It works for me.
1 small bottle of tea tree oil.
1 300 mL spray bottle of 99% isopropanol from the drug store. I could only find a link to the 70% but it's the same bottle and brand.
Spray I use
pour out a little alcohol, pour in the tea-tree oil and shake.
It works great, doesn't seem to separate like water and tea-tree oil! Spray in all the lockers and the bilge. Might not be as good as a gel for some things, but it coats everything with a light coat of tea-tree oil, and I think the alcohol helps dry stuff out and kill things too. I've used a quick spray on cuts as well, when I had nothing else.